THE PDGF RECEPTOR PHOSPHORYLATES TYR-138 IN THE C-SRC SH3 DOMAIN IN-VIVO REDUCING PEPTIDE LIGAND-BINDING

Treatment of quiescent NIH3T3 cells with PDGF BE results in the transi ent activation and hyperphosphorylation of the protein-tyrosine kinase , c-Src. These effects correlate with novel serine and tyrosine phosph orylations in the N-terminal non-catalytic region of the molecule, whi ch contains an SH3 and SH2 domain. In this study, a site of PDGF-induc ed tyrosine phosphorylation was mapped to Tyr 138 in the SH3 domain; T yr 138 is exposed on the SH3 peptide binding surface. This same site i s phosphorylated in vitro by the PDGF receptor when purified baculovir us-expressed c-Src is complexed with the activated receptor. Phosphory lation of Tyr 138 required association of c-Src with the PDGF receptor via its SH2 domain. When a c-Src Phe 138 mutant was stably expressed in Src(-) mouse fibroblasts, it was activated to the same extent as wi ld type c-Src following PDGF stimulation, indicating that phosphorylat ion of this site is not required for PDGF-mediated activation. However , Tyr 138 phosphorylation was found to diminish SH3 domain peptide lig and binding ability in vitro.